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參數(shù)資料
| 型號： | LM4894MWC |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 1 W, 1 CHANNEL, AUDIO AMPLIFIER, UUC |
| 封裝： | WAFER |
| 文件頁數(shù)： | 4/21頁 |
| 文件大?。?/td> | 781K |
| 代理商： | LM4894MWC |
Application Information (Continued)
mise the LM4894’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The LD package must have its DAP soldered to a copper
pad on the PCB. The DAP’s PCB copper pad is connected to
a large plane of continuous unbroken copper. This plane
forms a thermal mass and heat sink and radiation area.
Place the heat sink area on either outside plane in the case
of a two-sided PCB, or on an inner layer of a board with more
than two layers. Connect the DAP copper pad to the inner
layer or backside copper heat sink area with 4 (2x2) vias.
The via diameter should be 0.012in - 0.013in with a 0.050in
pitch. Ensure efficient thermal conductivity by plating-
through and solder-filling the vias.
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. If the heatsink and amplifier
share the same PCB layer, a nominal 2.5in
2 (min) area is
necessary for 5V operation with a 4
load. Heatsink areas
not placed on the same PCB layer as the LM4894 should be
5in
2 (min) for the same supply voltage and load resistance.
The last two area recommendations apply for 25C ambient
temperature. In all circumstances and conditions, the junc-
tion temperature must be held below 150C to prevent acti-
vating the LM4894’s thermal shutdown protection. The
LM4894’s power de-rating curve in the Typical Performance
Characteristics shows the maximum power dissipation ver-
sus temperature. Example PCB layouts for the exposed-
DAP TSSOP and LLP packages are shown in the Demon-
stration Board Layout section. Further detailed and specific
information concerning PCB layout, fabrication, and mount-
ing an LLP package is available from National Semiconduc-
tor’s package Engineering Group under application note
AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3
AND 4 LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1
trace resistance reduces
the output power dissipated by a 4
load from 1.4W to
1.37W. This problem of decreased load dissipation is exac-
erbated as load impedance decreases. Therefore, to main-
tain the highest load dissipation and widest output voltage
swing, PCB traces that connect the output pins to a load
must be as wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup-
plies, trace resistance creates the same effects as poor
sup-ply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifer, whether the amplifier is bridged or
single-ended. Equation 2 states the maximum power dissi-
pation point for a single-ended amplifier operating at a given
supply voltage and driving a specified output load.
P
DMAX=(VDD)
2/(2
π2R
L) Single-Ended
(2)
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is an increase in
internal power dissipation versus a single-ended amplifier
operating at the same conditions.
P
DMAX = 4*(VDD)
/(2
π2R
L) Bridge Mode
(3)
Since the LM4894 has bridged outputs, the maximum inter-
nal power dissipation is 4 times that of a single-ended am-
plifier. Even with this substantial increase in power dissipa-
tion, the LM4894 does not require additional heatsinking
under most operating conditions and output loading. From
Equation 3, assuming a 5V power supply and an 8
load,
the maximum power dissipation point is 625mW. The maxi-
mum power dissipation point obtained from Equation 3 must
not be greater than the power dissipation results from Equa-
tion 4:
P
DMAX =(TJMAX -TA)/
θ
JA
(4)
The LM4894’s
θ
JA in an MUA10A package is 190C/W.
Depending on the ambient temperature, T
A, of the system
surroundings, Equation 4 can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation 3 is greater than that of Equation 4,
then either the supply voltage must be decreased, the load
impedance increased, the ambient temperature reduced, or
the
θ
JA reduced with heatsinking. In many cases, larger
traces near the output, V
DD, and GND pins can be used to
lower the
θ
JA. The larger areas of copper provide a form of
heatsinking allowing higher power dissipation. For the typical
application of a 5V power supply, with an 8
load, the
maximum ambient temperature possible without violating the
maximum junction temperature is approximately 30C pro-
vided that device operation is around the maximum power
dissipation point. Recall that internal power dissipation is a
function of output power. If typical operation is not around the
maximum power dissipation point, the LM4894 can operate
at higher ambient temperatures. Refer to the Typical Per-
formance Characteristics curves for power dissipation in-
formation.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection ratio (PSRR). The capacitor location on both the
bypass and power supply pins should be as close to the
device as possible. A larger half-supply bypass capacitor
improves PSRR because it increases half-supply stability.
Typical applications employ a 5V regulator with 10F and
0.1F bypass capacitors that increase supply stability. This,
however, does not eliminate the need for bypassing the
supply nodes of the LM4894. Although the LM4894 will
operate without the bypass capacitor C
B, although the PSRR
may decrease. A 1F capacitor is recommended for C
B. This
value maximizes PSRR performance. Lesser values may be
LM4894
www.national.com
12
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